1. Field of the Invention
This invention relates generally to thermal heads which are widely used in facsimile machines, word processors and so forth. More specifically, the invention relates to a method of making unit heads which are arranged in series in an optional number to constitute a divisional-type thermal head of a desired length.
2. Description of the Prior Art
As is well known, thermal heads are widely used to print information on papers of various sizes. Thus, the length of the thermal head must be adjusted depending on the particular paper size to which the thermal head is applied.
However, if the paper size is too large, it becomes difficult or impractical to increase the length of a single thermal head to suit the excessively large paper size. Further, it is technically disadvantageous to provide thermal heads of various sizes due to the necessity of redesigning upon every change in size.
In view of the above problem, it has been proposed to constitute a thermal head of a desired length by incorporating a series of standardized unit heads in a selected number. Such a thermal head is called "divisional-type thermal head".
For convenience of explanation, an example of divisional-type thermal head is shown in FIG. 7 of the accompanying drawings, whereas a unit head to be incorporated in the divisional-type thermal head is illustrated in FIG. 8.
As shown in FIG. 7, the divisional-type thermal head has a support plate B on which a plurality of unit heads A" (see FIG. 8) are arranged in series. Each unit head A" has a length (unit printing length) s, so that the divisional-type thermal head provides an overall printing length S which is a multiple of the unit printing length s. Therefore, the overall printing length S can be optionally adjusted by selecting the number of unit heads A" without changing the length of each unit head itself.
A method of making such unit heads is disclosed in Japanese Patent Publication No. 63-11993 for example. For convenience, the method disclosed in this publication is illustrated in FIG. 9 of the accompanying drawings.
As shown in FIG. 9, a material head A0 is first prepared for subsequent making of unit heads A" (see also FIG. 8). The material head comprises an elongate insulating substrate 1 having a length L and a width W, and a longitudinal array of heating dot portions 3 formed on the substrate 1 and respectively connected to pairs of electrodes 4, 5. The material head further comprises a protective layer 6 extending longitudinally of the substrate 1 to cover the heating dot portions 3 and the electrodes 4, 5.
Then, the material head A0 is cut along cutting lines C each extending transversely of the substrate 1 at a position between two adjacent heating dot portions. The cutting may be performed by using a rotary diamond cutter (not shown) for example. As a result, a plurality of unit heads A" are provided for subsequent incorporation into a divisional-type thermal head.
The protective layer 6 of the material head A0 is usually made of a hard but brittle material such as glass. Thus, at the time of cutting the material head, the protective layer is inevitably subjected to chipping at the cut edges extending along the respective cutting lines C. If such chipping is excessive, the heating dot portions 3 and electrodes 4, 5 located near the cutting lines may partially come off with the chips. Further, similar chipping also occurs during subsequent handling and transportation of the divided unit heads A". Therefore, the unit heads are liable to a quality problem.
Obviously, if the width of the protective layer 6 is rendered small, the degree of chipping reduces. However, such width reduction detracts from the protective function of the protective layer.
Moreover, when the substrate 1 itself is formed with a glaze layer 2 of e.g. glass (as often required for surface conditioning), the problem of chipping also occurs with respect to the glaze layer. Indeed, the glaze layer 2 will have a width corresponding to that of the substrate 1, so that the chipping problem is additionally serious.